op-amp config

[zesla]

Hi,

Please tell me if the below circuits are satisfied at audio frequncies?

I looked at the datasheet of the TL072 but I was not able to find out if this config and specially those two gains showed below satisfy the TL072 main parameters?

Actually I do not know which parametrs should be considered other than the power supply volatge?

 

[zesla]

by the way any idea how to have the 180 degree of the output signal? I think adding another op-amp on inverse config would do so but do not know the actual configuration.

 

[zesla]

No response?

Please at least tell me which parameters should be considered when changing the gain by R11 & R5?

 

[Dragon Tamer]

Your gain is way too high. For an op-amp to amplify an audio audio signal, then you need to have an op-amp with a gain of 11 or less. Use a 10k feedback resistor (R11) and connect another 10k resistor (R5) to ground. To find out the minimum input voltage for your op-amp it would be found in the data sheet as the differential input voltage.

 

[zesla]

Your gain is way too high. For an op-amp to amplify an audio audio signal, then you need to have an op-amp with a gain of 11 or less. Use a 10k feedback resistor (R11) and connect another 10k resistor (R5) to ground. To find out the minimum input voltage for your op-amp it would be found in the data sheet as the differential input voltage.

Well, gain of 11 is too low for me, What if the gain incrases?!
The above config would be used as a mike amplifier, so I need a gain of more than 100. What will happen if I incrase the gain? Any problem?

I did not find "differential input voltage" in the datsheet. What it does state really?

 

[audioguru]

Your first opamp has a gain of 101 (1M/10K +1). Its 10k resistor and 1uf capacitor to ground reduces the gain of frequencies below 16Hz.
The datasheet has a graph shoing the high frequency rolloff of high frequencies when the gain is changed. With a gain of 101 then its high frequencies are reduced above 30kHz.

Your second opamp has a gain of 471 (470k/1k +1). Its high frequencies are reduced above 6400Hz.

Your circuit is non-inverting. If you want it to invert then make an inverting opamp circuit.

 

[audioguru]

you need to have an op-amp with a gain of 11 or less. Use a 10k feedback resistor (R11) and connect another 10k resistor (R5) to ground.

Then the gain is only 2. Almost like a piece of wire.

 

[Dragon Tamer]

Well, gain of 11 is too low for me, What if the gain incrases?!
The above config would be used as a mike amplifier, so I need a gain of more than 100. What will happen if I incrase the gain? Any problem?

I did not find "differential input voltage" in the datsheet. What it does state really?

Well first I'd like to appalogize, it wasn't the differential input voltage, it was the input offset voltage, which was between 3mV and 10mV. So that would be your minimum input voltage. You may need to make a preamp before connecting the mic to the op-amp.

I connected my mic to one of my TL082 op-amps and I had no noticeable problems with a gain of 6.

 

[Electronman]

Your first opamp has a gain of 101 (1M/10K +1). Its 10k resistor and 1uf capacitor to ground reduces the gain of frequencies below 16Hz.
The datasheet has a graph shoing the high frequency rolloff of high frequencies when the gain is changed. With a gain of 101 then its high frequencies are reduced above 30kHz.

Your second opamp has a gain of 471 (470k/1k +1). Its high frequencies are reduced above 6400Hz.

Your circuit is non-inverting. If you want it to invert then make an inverting opamp circuit.

Hi audioguru,

Thanks for your reply, you are always very helpful.

Well first of all please respond to a question I have dealing with for a while, here's it is:
You told that roll-off frequency (do you call it so here?) is when the resistance of the resistor is equal to the reactance of the cap so you told Fc=1/(2pi x 1k x 1u). The question I am dealing with is that why the roll-off frequency (16Hz for here) is when the R5=Xc??? Whats the main reason for it please?

Which graph are you refereeing to? I can not see any graph showing the high frequency roll-off of high frequencies when the gain is changed??

So is there any other main parameter I should consider yet?
What about Slew rate?

Thanks

P.S, How to connect a inverting op-anp to it so that I get max accuracy? I want the ONLY diference between the inverting and the noninverting op-amp be the 180 degree phase shift, the gain must be JUST like the other half.

 

[audioguru]

Well first I'd like to appalogize, it wasn't the differential input voltage, it was the input offset voltage, which was between 3mV and 10mV. So that would be your minimum input voltage.

No.
A mic preamp amplifies the AC signal, not the DC odffset voltage. Mic preamps work with input signal levels of microvolts and the noise level is what limits the smallest signal level.

You may need to make a preamp before connecting the mic to the op-amp.

No.
The opamp is the preamp.

I connected my mic to one of my TL082 op-amps and I had no noticeable problems with a gain of 6.

Low level audio creates a signal of only microvolts at the output of a mic so the gain must be 200 to 300.
Your gain of only 6 is used when you are screaming at a mic that is touching your lips or inside a drum.

 

[Electronman]

Well first I'd like to appalogize, it wasn't the differential input voltage, it was the input offset voltage, which was between 3mV and 10mV. So that would be your minimum input voltage. You may need to make a preamp before connecting the mic to the op-amp.

I connected my mic to one of my TL082 op-amps and I had no noticeable problems with a gain of 6.

Sorry but I completely do not understand you. Maybe it is due to Language barriers?
Can you please enlighten me of what you are talking about?

thanks

 

[zesla]

Your second opamp has a gain of 471 (470k/1k +1). Its high frequencies are reduced above 6400Hz.

Thanks,

So for this gain I have to choose abother OP-amp.
What about AD826? Is it able to handel a such gain at audio frequncies? It is not Low noise like TL072 though.

 

[audioguru]

You told that roll-off frequency (do you call it so here?) is when the resistance of the resistor is equal to the reactance of the cap so you told Fc=1/(2pi x 1k x 1u). The question I am dealing with is that why the roll-off frequency (16Hz for here) is when the R5=Xc??? Whats the main reason for it please?

A 1uF capacitor has a reactance of 10k ohms at 16Hz. But the gain of the opamp is Rf/R to ground. R to ground is 10k ohms. The reactance of the capacitor is increasing R to ground so the gain is -3dB at 16Hz.

Which graph are you refereeing to? I can not see any graph showing the high frequency roll-off of high frequencies when the gain is changed??

All opamps have a graph that shows its max gain at various frequencies and at different gains. The opamp has built-in frequency compensation that rolls off the high frequencies so the gain is less than 1 at a high frequency where phase shifts inside the opamp add to 180 degrees. Since the gain is less than 1 then when negative feedback is added then the opamp will not oscillate at the 180 degrees phase shifted frequency.

Here is the graph from the TL072 datasheet. It shows a max gain of 100,000 up to a frequency of 32Hz. The max gain is 10,000 up to 320Hz. The max gain is 1,000 up to 3.2kHz. The max gain is 100 up to 32kHz. The max gain is 10 up to 320kHz. The gain is 1 at 3.2MHz.

What about Slew rate?

The slew rate is shown on another graph on most opamps' datasheets. It is the frequency where the output of an opamp cannot slew fast enough to produce a sine-wave and instead produces a triangle-wave with the output level dropping if the frequency is higher.
A lousy old LM324 or LM358 has trouble above only about 2kHz. A lousy old 741 opamp has trouble above only 9kHz. The TL072 works perfectly up to 100kHz.

How to connect a inverting op-anp to it so that I get max accuracy? I want the ONLY diference between the inverting and the noninverting op-amp be the 180 degree phase shift, the gain must be JUST like the other half.

You can adjust the gain of an inverting opamp to be the same as the gain of a non-inverting opamp. You must be aware that the inverting opamp has a much lower input resistance. A differential amplifier uses two or three opamps so that both inputs have a very high input resistance.

 

[audioguru]

So for this gain I have to choose abother OP-amp.
What about AD826? Is it able to handel a such gain at audio frequncies? It is not Low noise like TL072 though.

An OPA134 is a single opamp. There are OPA2134 duals and OPA4134 quads. They are very low noise and very low distortion.
Their datasheet shows that with a gain of 471 their gain drops above about 18khz.

If you want a flat frequency response to a higher frequency then use two opamps in series and each one has less gain but their total has plenty of gain.

 

[Jaguarjoe]

If you want a flat frequency response to a higher frequency then use two opamps in series and each one has less gain but their total has plenty of gain.

Can't you take the square root of the total desired gain and use that value to be the gain of each of the two amplifiers? That would provide the maximum available frequency response for a particular total gain.

 

[audioguru]

Can't you take the square root of the total desired gain and use that value to be the gain of each of the two amplifiers? That would provide the maximum available frequency response for a particular total gain.

Correct and I need to add enough words so this answer can be posted.

 

[zesla]

Can't you take the square root of the total desired gain and use that value to be the gain of each of the two amplifiers? That would provide the maximum available frequency response for a particular total gain.

What do you mean by that & how to do so?

 

[audioguru]

What do you mean by that & how to do so?

An opamp with high gain has a poor high frequency response but if the gain is less then the high frequency response is good.

So simply add two opamps in series so that their total gain is what you want then the high frequency response is much better than if only one opamp is used.

You take the square-root of the total gain to find the gain for each opamp since the gains multiply.
Example: Opamp #1 has a gain of 20 and opamp #2 also has a gain of 20. The total gain is 400 and its square-root is 20.

 

[zesla]

An opamp with high gain has a poor high frequency response but if the gain is less then the high frequency response is good.

So simply add two opamps in series so that their total gain is what you want then the high frequency response is much better than if only one opamp is used.

You take the square-root of the total gain to find the gain for each opamp since the gains multiply.
Example: Opamp #1 has a gain of 20 and opamp #2 also has a gain of 20. The total gain is 400 and its square-root is 20.

Yea good idea, Thanks for it. I know it as Cascading.

Ok audioguru, I want to do so but which cobgigration is the best choice to deal with? I want my config (at the first post) to be the secod stage, But not sure which configuration is the best choice for the first stage? My electeret Mike has a insuded battery for its bias too.
The other pont is that I need to provide a DC offset in the input for some reason (I did it by a 10K pot on the non inverting of my circuit which is showed at post NO 1).

Thanks for any suggestion or any circuit.

 

[zesla]

Can I done it by cascading just 2 similar configs I showed in my first post?